1. Field of the Invention
This invention relates generally to radio frequency phase shifting circuits and more particularly to a radio frequency circuit that provides a 180.degree. phase shift to a signal propagated therethrough.
2. Description of the Prior Art
Phase shifters are devices in which the phase of an electromagnetic wave of a given frequency propagating through a transmission line can be shifted. Such phase shifters are utilized in many microwave systems and in particular are required for electronic beam steering and phased array radar systems. A typical prior art wide band 180.degree. phase shifter bit is constructed by placing switching diodes of the coupled and through ports of a branch line or Lange coupler. By switching the diodes off and on, the signal appearing at the output ports of the coupler exhibit a relative 180.degree. phase shift. Such a phase shifter is limited to generally less than an octave bandwidth and microwave frequency because of phase shift deviations from the desired 180.degree..
Other types of phase shifters are found in the prior art in which a change in phase is obtained by utilizing one of a number of lengths of transmission line to approximate the desired value of phase change. Various lengths of transmission line are inserted and removed by high speed electronic switching. Semiconductor diodes and ferrites are the devices commonly employed in digital phase shifters. One such digitally switched phase shifter is a parallel line configuration in which the proper transmission line segment is selected from among many available parallel lines. An alternative phase shifter is a series line or a cascaded multi-bit digitally switched phase shifter.
Another type of phase shifter is described in U.S. Pat. No. 4,612,520 to Boire et al. This phase shifting bit utilized switching means to alternatively connect the RF input and RF output to a coupled transmission line segment and a pi network segment. However, when a standard -3 db, 50 ohm coupler is utilized in this design to achieve a wide bandwidth, the circuit experiences relatively high (approximately 3 to 4 db) insertion loss. This is because this design requires very small series switches in order to keep the capacities of the switches small so that no RF signal will travel through the "off" path of the circuit. And if different coupling or impedance is chosen, a VSWR mismatch develops.